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ItemEffects of microstructure on the susceptibility of austenitic and martensitic stainless steels to pitting and intergranular corrosion in aqueous chloride environments(Georgia Institute of Technology, 2012-05-07) Sikri, Tarun Vabhav PratapStainless steels are utilized for their high toughness and resistance to general corrosion. Austenitic (300 series) stainless steels are the most popular because they are ductile and can be easily formed into desired geometries. They can also be case hardened to form alternating layers of martensitic and austenitic microstructures for applications that require high toughness and resistance to surface wear. However their usage is limited in comparison to other ferrous alloys due to higher initial costs and susceptibility to pitting and intergranular (IGC) corrosion. A microcell was developed to study these localized corrosion phenomena in microstructural regions of interest by performing polarization (spot) tests within well defined areas on metallic surfaces. Spot tests across profiles of welded 304 stainless steel confirmed that sensitization, greater acidities and higher chloride contents increase susceptibility and greater additions of chromium and nickel reduce susceptibility to localized corrosion. Spot tests across a case hardened (CSS 42L) stainless steel profile revealed that the austenitic sensitized outer layer was more susceptible to localized attack compared to the martensitic matrix. A more complete understanding of how microstructure affects these localized corrosion processes will lead to better alloy modifications, service environments and maintenance making this class of material a more sustainable alternative.
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ItemA molecular dynamics simulation study of oxygen within hydrated nafion-117 for fuel cell application(Georgia Institute of Technology, 2012-05-07) Fuller, JeffreyFuel cells have several obstacles before they can see widespread use. Nafion, the current most successful polymer for use in polymer electrolyte fuel cells (PEMFCs) has been characterized by numerous other studies both experimental and computational. This study determines the, until now unexplored, behavior of oxygen within a Nafion system hydrated at 10 and 20wt%. The systems are equilibrated at temperatures of 353, 363, 373, and 383 Kelvin. Then the structure and transport characteristics of the system are explored using pair correlation and mean square displacement functions. As the hydration increased, the oxygen molecules became more coordinated with water resulting in a decrease in the diffusion coefficient of oxygen within the system. This has important implications in the efficiency of the fuel cell. This data furthers the pursuit of a more perfect polymer membrane for use in fuel cells.
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ItemInvestigating steric protection of DNA in the presence of nucleases(Georgia Institute of Technology, 2011-05-11) Tomassi, Taylor AlexandraIn the human body, DNA-linked colloidal assemblies are prone to cleavage by nucleases, yielding the uncontrolled release of particles and any associated therapeutics. Thus, for in vivo applications, the DNA-linkages must be protected from cleavage by serum nucleases. The goal of this research is to stabilize DNA duplexes in the presence of nucleases by chemically modifying the primary target. The effects of sterically protecting DNA duplexes from nuclease activity by including a polymeric tail on oligonucleotide targets and by including LNA bases in the target sequence were investigated. The variables explored included the effect of tail chemistry as well as tail length on the kinetics and extent of nuclease activity. In DNA digests, two types of polymeric "tails" were compared: polyethylene glycol (PEG) chains and single stranded thymine-based strands (dT). Long and short PEG and thymine tails of equivalent lengths were compared. Ezymatic digests were also performed on fluorescently labeled primary targets modified with oligonucleotide analogs called LNA, locked nucleic acids. Flow cytometry was used to quantify the hybridization activity and measure the probe-target duplex density as well as to determine time-dependence of nuclease activity by monitoring the number of duplexes remaining following incubation with DNase I. Significant clipping was observed for all DNA targets tested and indicated that various polymeric tails did not significantly hinder nuclease activity. These results indicate that the relatively short polymeric tail lengths do not have appreciable effects on the hindrance of nuclease activity. LNA digests, on the other hand, showed enhanced stability of primary duplexes in the presence of nucleases after 24 hours and suggested that LNA may be used as an alternative to DNA to stabilize colloidal assemblies for drug delivery.
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ItemAmorphous silicon and carbon nanotube-based photovoltaics(Georgia Institute of Technology, 2010-12-20) Nguyen, Justin J.In this study, the possibilities of creating an effective amorphous silicon (a-Si) based photovoltaic (PV) cell supported by a 3D carbon nanotube (CNT) structure are investigated. Vertically aligned carbon nanotubes (VACNTs) provide a significant advantage increasing photon dwell time in photovoltaic materials via light trapping at off-normal angles to the sun. By creating a 3D array, CNTs allow for the use of less and therefore cheaper PV material, particularly a-Si. This study examines and presents the creation of such a novel PV cell through thermally-enhanced chemical vapor deposition (TECVD) and plasma-enhanced chemical vapor deposition (PECVD) techniques. The 3D a-Si PV cell that is presented consists of a single junction p-i-n photodiode less than 350 nm in total width that absorbs photons just above the 1.7 eV energy level. A close examination of the growth and morphology of a-Si and nanocrystalline silicon (nc-Si) thin films on CNTs is also presented and studied, with discussion stemming from the results of scanning electron microscopy (SEM), optical microscopy, atomic force microscopy (AFM), Raman spectroscopy, x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Vis spectrophotometry, and current-voltage measurements.
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ItemBaF₂:Ce Polymer Composite Gamma-ray Scintillators(Georgia Institute of Technology, 2010-05-12) Barta, M BrookeA nanocomposite gamma-ray scintillator contains nanoscale phosphor particles (nanophosphors) encapsulated in a polymer matrix. These scintillators show great promise for improving the cost and durability of scintillators for gamma-ray detector systems because single crystal scintillators currently in use have high cost and hygroscopicity, and are available in limited compositions and sizes. This research utilizes two different commercially available epoxies to encapsulate two different nanophosphors (BaF2:2%Ce and BaF2:15%Ce). The importance of three parameters are illustrated: photon efficiency of the encapsulated nanophosphor, refractive index (RI) match between the nanophosphor and polymer matrix, and the %transmission of the polymer at the wavelength at which the BaF2:Ce nanophosphors emit gamma-ray excited photons.
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ItemThe Path to Graphene Synthesis and Applications(Georgia Institute of Technology, 2010-05-12) Soracco, DavidGraphene is an amazing novel material consisting of one atomic layer of carbon in a hexagonal lattice. The crystal is incredibly versatile, exhibiting useful electrical, thermal, optical, quantum, mechanical and chemical properties. The state of progress in the microelectronics industry is following a constant trend now, but limits on feature miniaturization are approaching in the next 10-20 years. Graphene is a novel material with very high measured carrier mobilities that stands to displace silicon as a viable material for integrated circuit fabrication. This project focuses on its growth by chemical vapor deposition over nickel. The objective is to reliably grow large area fractions of mono and bilayer graphene samples through controlled precipitation of carbon in 300-500 nm nickel films. Samples are analyzed by Raman spectroscopy and sheet resistance measurements as well as optical microscopy. While successful mono and bilayer graphene sample growth is not confirmed, the results echo similar conclusions as recent research into the same process.
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ItemInvestigating steric protection of DNA in the presence of nucleases(Georgia Institute of Technology, 2010-05-11) Tomassi, Taylor AlexandraIn the human body, DNA-linked colloidal assemblies are prone to cleavage by nucleases, yielding the uncontrolled release of particles and any associated therapeutics. Thus, for in vivo applications, the DNA-linkages must be protected from cleavage by serum nucleases. The goal of this research is to stabilize DNA duplexes in the presence of nucleases by chemically modifying the primary target. The effects of sterically protecting DNA duplexes from nuclease activity by including a polymeric “tail” on oligonucleotide targets and by including LNA bases in the target sequence were investigated. The variables explored included the effect of tail chemistry as well as tail length on the kinetics and extent of nuclease activity. In DNA digests, two types of polymeric "tails" were compared: polyethylene glycol (PEG) chains and single stranded thymine-based strands (dT). Long and short PEG and thymine tails of equivalent lengths were compared. Ezymatic digests were also performed on fluorescently labeled primary targets modified with oligonucleotide analogs called LNA, locked nucleic acids. Flow cytometry was used to quantify the hybridization activity and measure the probe-target duplex density as well as to determine time-dependence of nuclease activity by monitoring the number of duplexes remaining following incubation with DNase I. Significant clipping was observed for all DNA targets tested and indicated that various polymeric tails did not significantly hinder nuclease activity. These results indicate that the relatively short polymeric tail lengths do not have appreciable effects on the hindrance of nuclease activity. LNA digests, on the other hand, showed enhanced stability of primary duplexes in the presence of nucleases after 24 hours and suggested that LNA may be used as an alternative to DNA to stabilize colloidal assemblies for drug delivery.